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In this work, comparative long-wave infrared (LWIR) laser-induced breakdown spectroscopy (LIBS) emission studies of two excitation sources: conventional 1.064 μm and eye-safe laser wavelength at 1.574 μm were performed to analyze several widely-used inorganic energetic materials such as ammonium and potassium compounds as well as the organic liquid chemical warfare agent simulant, dimethyl methylphosphate (DMMP). LWIR LIBS emissions generated by both excitation sources were examined using three different detection systems: a single element liquid nitrogen cooled Mercury Cadmium Telluride (MCT) detector, an MCT linear array detection system with multi-channel preamplifiers + integrators, and an MCT linear array detection system with readout integrated circuit. It was observed that LWIR LIBS studies using an eye-safe pump laser generally reproduced atomic and molecular IR LIBS spectra as previously observed under 1.064 µm laser excitation.
© 2017 Optical Society of America
1. Introduction
Nowadays, laser-based detection techniques are being investigated to detect or identify explosives or chemical agents in response to the increasing threat to international security. Laser-induced breakdown spectroscopy (LIBS) has proven to be a versatile and sensitive technique used for detection of trace substances as well as the analysis of the elemental composition of materials [1–8]. LIBS research has garnered great attention in the field of instrumentation and data analysis techniques by virtue of its wide range of applications in standoff detection of chemical, biological, and explosive (CBE) materials, and remote sensing. The atomic LIBS emissions have been extensively used to analyze substances such as metallic, non-metallic, liquid, gases, aerosol, explosives, and biological materials [1–14]. LIBS relies on the short-lived micro-plasma generated by an intense laser pulse to dissociate, atomize, and ionize target molecules. In the ultraviolet–visible (UV-Vis) spectral region, those breakdown products emit intense emission lines characteristic of electronic transitions of atoms, ions, and small molecular fragments (e.g. OH, CN) [2,15]. Important information on the identification and concentration of the trace materials can be derived from the analysis of LIBS emission spectra.
Besides those elemental emissions in conventional UV-Vis LIBS, molecular vibrational LIBS emissions of the target compounds were clearly observed in the long-wave infrared (LWIR) region [16–27]. Recent studies of LIBS emissions in the mid- to long-wave infrared (2-12 µm) region readily observed and identified several emitting atomic and complex molecular species resulting from the laser-induced micro-plasma formation: neutral metal atoms [17,18], oxygenated combustion molecular byproducts (e.g. CO2, H2O) [16,18–21], and intact sample molecules [19–27]. Intense and distinct atomic and molecular LWIR emission signatures of wide variety of solid inorganic and organic tablets [16–24] as well as thin liquid and solid films of organic surface contaminations [23,26,27] such as explosives, chemical, and biological warfare agent simulants, deposited on surface of real-life substrates are readily observed. The LWIR LIBS emission spectra of the substrate materials, such as metals, concrete, asphalt and sands, in those surface contamination studies were found to be generally broad and easily distinguished from the LWIR LIBS signature emissions of the surface contaminants. Intact sample target molecules not only survived the laser ablation, but were excited by the laser photons and/or induced plasmas. Previous studies using a MCT linear array detection system with 1.064 μm exciting laser estimated the limit of detection (LOD) to be in the order of microgram per square centimeter for solid thin film and microliter per square centimeter for liquid thin film on surfaces of various substrates [26,27]. The combination of atomic emission signatures derived from conventional UV-Vis LIBS and fingerprint signatures of intact target molecular entities determined from LWIR LIBS may become a more powerful spectral tool for chemical detection and analysis.
Most current LIBS setups and devices employ a fundamental Q-switched Nd:YAG laser operating at 1.064 μm, which has an extremely low threshold for eye-damage. Laser wavelengths longer than ~1.4 μm are often called “eye-safe” or particularly “retina-safe” since they are not well transmitted within the ocular media and are poorly absorbed by the retina [28]. This work is the first comprehensive study of LIBS emissions in the LWIR region excited by an eye-safe (~1.574 μm) laser source. In this work, comparative LWIR-LIBS emission studies of these two excitation sources: conventional 1.064 μm and eye-safe 1.574 μm laser wavelengths were performed to study several widely-used inorganic energetic materials such as ammonium and potassium compounds as well as the liquid chemical warfare agent simulant, dimethyl methylphosphate (DMMP).
2. Experimental details
Previous molecular vibrational LIBS emissions in the long-wave infrared (LWIR) region have been studied using three different LWIR LIBS detection systems: a single element liquid nitrogen cooled Mercury Cadmium Telluride (MCT) detection system [16–22], an MCT linear array detection system with multi-channel preamplifiers + integrators [25], and an MCT linear array detection system with a readout integrated circuit [23–27]. The configuration and calibration of these LWIR LIBS MCT detection systems are described in detail in previous studies [16,23,25]. Basically they all consist of a short pulsed pumping laser source, a set of optics to focus a short laser pulse onto a vertically mounted condensed-phase sample, a translational sample stage, a set of collection optics to collect and focus the LIBS emission signals on the entrance slit of a monochromator, a monochromator, an MCT detector and read out electronics to temporally gate the detector and record the signals. The samples were translated during measurements so that every laser pulse would hit a fresh sample spot.
It is our intention to evaluate the performance of eye safe laser pumping pulse in all three LWIR LIBS detection systems known to date. Comparative LWIR-LIBS emission studies in this work used the experimental setup schemes of those three LWIR LIBS detection systems in the previous studies [16–27]. For example, a schematic diagram of the experimental setup of single element liquid nitrogen cooled MCT detection system for comparative LWIR-LIBS emission studies in this work is illustrated in Fig. 1. It is very similar to the ones used in previous 1.064 μm studies [16–22]. The solid samples were mounted on a linear translation stage which was attached to a stepper motor with a motion controller. The laser beam was focused with a lens (focal length = 7.5 cm) onto the sample surface leading to a beam diameterfocused with a lens (focal length = 7.5 cm) onto the sample surface leading to a beam diameter of ~0.2 mm. Two ZnSe lenses (focal length = 10 cm) were employed as the collection optics to focus the sample LIBS emission onto the entrance slit of a 0.15 m spectrometer. The spectrometer was equipped with 150 grooves/mm and 75 grooves/mm reflecting grating blazed at 4 μm (MIR) and 8 μm (LWIR), respectively. Long pass filters for corresponding spectral regions were utilized to block laser scattering and the shorter emission wavelengths. The emission spectra were recorded using a liquid-nitrogen cooled single element HgCdTe (MCT, 2-12 μm) detector. The sample size is usually 15 mm diameter and each emission scan takes ~12 minutes for a wavelength range of 1200 nm. The emission signals were averaged using a boxcar averager with a gate width of 16 µs and delay time of 20 µs. The only difference is the exciting laser source employed in the emission studies. All the previous LWIR LIBS studies employed a fundamental flash lamp pumped Q-switched Nd:YAG laser either from Continuum (Surelite II-10) or Quantel Laser operating at 1.064 μm as the short pulsed (pulse width of 5-10 ns, 10 Hz repetition rate) pumping laser source [16–24]. 1.064 μm emission studies in this comparative work used a Continuum (Surelite II-10) laser operating at 1.064 μm. Eye-safe laser excitation emission studies in this comparative work employed a compact Nd:YAG (Quantel) pumped Optical Parametric Oscillator (OPO) laser system operating at 1.574 μm. The 1.574 μm laser output has a beam diameter of ~6.5 mm and pulse width of 11 ns with repetition rate of 10 Hz. The energy per laser pulse on the sample was set to ~70 mJ for both laser sources. These Q-switched Nd:YAG laser (1.064 μm) and OPO laser outputs (1.574 μm) were utilized in comparative LWIR-LIBS emission studies of the other two LWIR LIBS array detection systems in the same way.
Fig. 1 A schematic diagram of the experimental setup for the LWIR LIBS studies using MCT single element point detection.
Powders of commercially available (Alfa Aesar) potassium compounds and ammonium compounds were pressed into sample pellets of ~2 cm diameter using a hydraulic press. Thin solid sample films of the inorganic energetic compounds on asphalt substrates were prepared by manually depositing powders of those compounds on low-heated asphalt. First, asphalt was heated to 30-40 °C for a few minutes and then removed from the heated surface. The sample powders were then distributed over on a slightly heated asphalt substrate and pressed down gently and carefully with a spatula. The sample and substrate were allowed to sit for at least 30 minutes before measurement to allow them to cool down to room temperature.
3. Results and discussion
3.1 LWIR LIBS studies of energetic materials and liquid chemical warfare agent simulants using MCT single element point detection
The distinct LWIR LIBS atomic and molecular emission signatures in the 4-12 µm region have been reported for several potassium, sodium, and ammonium compounds due to dissociated and/or recombined molecular fragments under conventional 1.064 µm excitation [16–24]. Figure 2 shows the LWIR LIBS emission spectra of ambient background air and two common inorganic energetic materials: ammonium nitrate (NH4NO3) and ammonium perchlorate (NH4ClO4), under 1.064 µm and 1.574 µm excitations. FTIR spectra of NH4NO3 and NH4ClO4 [29,30] illustrated in Fig. 2(a) and 2(b) (dotted lines) confirm the molecular emission fingerprints observation of the investigated materials using either laser excitation wavelength. It can be seen that LWIR LIBS emission spectra of both chemicals in Fig. 2 show close agreement with their respective FTIR spectral bands. Infrared LIBS signatures for ammonium deformation band at ~7.0 µm as well as the asymmetric stretching bands of the NO3 (~7.45 µm) and ClO4 (~9 µm) anions [19–21,31] were observed under both laser excitations [Fig. 2(a) and 2(b)]. Despite the similar laser pulse energy, the LIBS emission intensity of ammonium compounds under 1.574 µm excitation appeared to be less than that under 1.064 µm pumping. As can be noted clearly in Fig. 2(c), the NO stretching band at ~5.2 µm [17–19] is pronounced in LIBS spectra excited under 1.064 µm while it is not noticeable in the 1.574 µm ones. Wong and Dagdigian [32] studied comparative LIBS emission of organic residues on aluminum substrates using 1.5 µm and 1.064 µm excitations in the UV-Vis spectral region. They observed that the overall visible LIBS emission intensity is considerably smaller at 1.5 µm than at 1.064 µm and also found significant differences in the intensity ratios of several emission features in the visible LIBS spectra at the two excitation wavelengths [32]. It was suggested that numerous factors could cause the weaker LIBS intensities at 1.5 µm, including less amount of material ablated into the plasma volume, higher plasma temperature, and higher electron density of the laser-induced plasma. The LIBS excitation and relaxation mechanisms of the sample atoms and small molecular fragments emitting in the visible region should be different from those of the intact sample and combustion byproduct molecules emitting in the LWIR region [17,24]. A reduced amount of material ablated into the plasma volume can still explain the emission intensity decrease from sample molecules excited by a 1.574 µm laser source in LWIR regions.
Fig. 2 LWIR LIBS emission spectra of ammonium nitrate and ammonium perchlorate pellets under 1.064 µm (grey dashed lines) and eye-safe laser at 1.574 µm (black solid lines) excitation. The FTIR absorption spectra are shown as dotted lines.
However, the disappearance of the combustion byproduct NO [19] could not be easily explained by any of the causes suggested in the visible LIBS study [32]. At higher plasma temperatures, the breakdown produced atoms of nitrogen and oxygen may form additional molecular products other than NO during relaxation processes. Detailed mechanism required further investigation.
Figure 3 shows the LWIR LIBS emission spectra of potassium nitrate (KNO3) and potassium chlorate (KClO3) pellets as well as sodium nitrate (NaNO3) and sodium chlorate (NaClO3) pellets measured under ambient air using 1.064 µm and 1.574 µm excitation. Under both excitation sources, potassium compounds showed distinct LWIR atomic emission signatures centered at ~6.3, ~7.5, and ~8.5 µm. The emission features were identified as the 6P1/2→4D3/2, 6H→ 5G, and 5D5/2 →6P3/2 inter-high-lying-Rydberg transition of neutral potassium atoms according to the NIST atomic spectra database [19–21, 33]. Several atomic emission features were also identified from sodium-containing compounds centered at ~4.66, 5.01, 5.43, 7.47, 9.12 µm which can be assigned to the following transitions of neutral sodium atoms: 72D3/2 → 52F5/2, 52D5/2 → 52P3/2, 52P3/2 → 52S1/2, 82F7/2 → 62D5/2, and 62D5/2 → 62P3/2, respectively [34]. In addition to the atomic LIBS emissions, characteristic molecular emissionsignatures of chlorate and nitrate were observed and identified for all materials. These observed molecular features showed strong resemblance with the asymmetric stretching bands in FTIR absorption spectra of the investigated materials (dotted line) [30]. Broad LIBS emission bands centered ~7.2 µm from KNO3 and NaNO3 as well as ~10.5 µm from KClO3 and NaClO3 thus can be attributed to the asymmetric stretching modes of molecular nitrate and chlorate moieties, respectively [19–21, 30]. It can be also noted that the emission feature of ambient air-oxidized nitrogen (NO) ~5.2 µm is not present in the LWIR LIBS under 1.574 µm excitation, as was mentioned earlier.
Fig. 3 LWIR LIBS emission spectra of (a) potassium nitrate (KNO3), (b) potassium chlorate (KClO3), (c) sodium nitrate (NaNO3), and sodium perchlorate (NaClO3), pellets under 1.064 µm (grey solid lines) and eye-safe laser at 1.574 µm (black solid lines) excitation. The FTIR absorption spectra are shown as dotted lines.
Comparative LWIR LIBS studies (4-12 µm) with different excitation wavelengths were also performed for the thin solid KClO3 and KClO4 films deposited on asphalt substrates. Asphalt is composed of two basic ingredients. First ingredients are the aggregates, which are a mixture of gravel and crushed limestone (calcium carbonate, CaCO3). The second is bitumen, which is a black sticky material that holds the aggregates together. Bitumen is comprised of polycyclic hydrocarbons (a petroleum byproduct). LIBS emissions observed from blank asphalt substrates (cut into ~2.5 cm in diameter and ~1.5 cm thickness and excited by both 1.064 µm and 1.574 µm) as well as CaCO3 (excited by 1.064 µm) are presented in Fig. 4. In asphalt substrate LWIR LIBS spectra, one can readily identify a broad emission feature around 6.5-7.5 µm, which corresponds most likely to a combination of the asymmetric stretching bands of CO3 from the limestone and C-H bending bands from the bitumen [35,36]. Figure 5 illustrates the LIBS emission spectra of asphalt substrates with thin film coating of KClO3 and KClO4 under 1.064 µm and eye-safe 1.574 µm excitation. Besides a broad emission band ranging ~6.5-8.0 µm from the asphalt substrate, both K atomic emission signatures as well as the asymmetric stretching bands of ClO3 and ClO4 anion [19–21] from the samples films were observed under both excitation sources.
Fig. 4 LWIR LIBS emission spectra of calcium carbonate (CaCO3, limestone) (black solid lines) along with FTIR absorption spectra of limestone (black dotted line) and bitumen (grey dotted line). LWIR LIBS emission spectra of bare asphalt substrates under 1.064 µm (grey dashed lines) and 1.574 µm (black dashed lines) pumping are also depicted.
Fig. 5 LWIR LIBS emission spectra of (a) potassium chlorate and (b) potassium perchlorate films on asphalt substrates under 1.064 µm (grey solid lines) and eye-safe laser at 1.574 µm (black solid lines) excitation. The FTIR absorption spectra of corresponding potassium compounds are shown as dotted lines.
Besides the identification and characterization of solid samples using LWIR LIBS techniques, LWIR LIBS emission signatures from liquid sample deposited on solid substrates have been studied and identified as well [23]. DMMP is the most commonly utilized simulant for Sarin nerve agent (GB, isopropyl methylphosphonofluoridate) due to its similar chemical properties. These CWAs can be obtained in solid, liquid, or vapor form. Both Sarin and DMMP are liquids at room temperature. In this work, liquid DMMP was used and ~0.2 ml was deposited on the surface of an asphalt substrate and let it sit for ~15 minutes before the thin liquid film of DMMP on asphalt substrate was seated and measured. Figure 6(a) and 6(b) illustrate the LWIR LIBS emissions of DMMP deposited on asphalt substrates measured using 1.064 µm and 1.574 µm excitation, respectively. FTIR absorption spectra reveal dominant emission signatures of the DMMP molecules between 5 and 12 µm: P-CH3 symmetric deformation band at ~7.6 µm, P = O stretching band at ~8.0 µm, O-CH3 rocking band at ~8.4 µm, and P-O-CH3 stretching band ~9.55 µm (Fig. 6, black dotted lines) [37,38]. LWIR LIBS spectra of DMMP on asphalt substrates using 1.064 µm and eye-safe 1.574 µm excitation sources both clearly showed emission features of DMMP molecules. As was described earlier, the broad features ~7 µm were from the asphalt substrates (Fig. 6, grey solid lines). The FTIR spectra of limestone, CaCO3 was also included for clarification (Fig. 6, black dashed lines).
Fig. 6 LWIR LIBS emission spectra of thin liquid DMMP film on the asphalt substrates (solid black lines) and bare asphalt substrates (grey solid lines) using 1.064 µm and eye-safe laser at 1.574 µm excitation. The FTIR absorption spectra of DMMP and limestone are shown as dotted lines and dashed lines, respectively.
3.2 LWIR LIBS studies using MCT array system
For intact target molecular species detected by a single element MCT LWIR LIBS detection system [19–22], it typically takes around 60 minutes to acquire a full spectrum from 4 to 12 µm by rotating the grating in the monochromator to scan all the wavelengths while advancing to avirgin area of the sample before generating the next LIBS pulse. Also due to this scanning requirement, the solid target sample must be prepared to have a uniform and homogenous surface. The inhomogeneity and roughness of the sample surface under investigation often lead to the lower signal to noise ratio in the spectra of the single element detection system. Therefore, to fulfill the promise of complementing conventional LIBS, the performance of IR LIBS must be improved. As result, two LWIR LIBS array detection systems: an MCT linear array detection system with multi-channel preamplifiers + integrators and an MCT linear array detection system with readout integrated circuit were developed to rapidly probe the samples and capture emission signatures [23–25].
In this section, LWIR LIBS studies from potassium compounds under 1.064 µm and 1.574 µm excitations were performed using these two MCT array detector systems. The MCT array detector in the multi-channel preamplifiers + integrators system was purchased from Infrared Systems Development Corporation [25]. It consists of a liquid nitrogen cooled, linear MCT array with 128 elements (photo diodes) in a single row configuration. The MCT array is coupled to a multi-channel preamplifier and high-speed integrator femtosecond pulse acquisition system. The MCT array also includes an electronic system which includes detector bias and preamplifiers, box-car type integrator, analog-to-digital converter, and read out electronics. All components are combined as a turn-key system that includes LabVIEW control and data acquisition software (National Instruments Corp., Austin, TX). Signal from each individual element (channel) is read out independently into a FIFO (first in first out). Then the FIFO is read out by the data acquisition software. The data acquisition software set and control the data integration gate, which determines the exact time the data are collected. All the channels (elements) share the same integration gate, which is triggered by the pumping laser pulse. The MCT detector array is coupled with a Horiba imaging spectrometer (0.19 m) equipped with a 3-grating turret. This system is capable of recording a wide range of LIBS emission spectra promptly, covering the spectral range from 2 to 12 µm (two turrets with interchangeable gratings). The current turret has 3 gratings: 4 µm (75 grooves/mm), 6 µm (50 grooves/mm), and 8 µm (30 grooves/mm). A schematic diagram of the experimental setup of LWIR LIBS emission setup using MCT multi-channel array detection system is shown in Fig. 7.
Fig. 7 A schematic diagram of the experimental setup for the LWIR LIBS studies using MCT multi-channel array detection system.
Applying 1.064 µm and eye-safe 1.574 µm excitations, the comparative LWIR LIBS measurements using the multi-channel preamplifiers + integrators MCT array detection were carried out for potassium compounds including KCl, KNO3, KClO3 and KClO4 pellets [Fig. 8(a) and 8(b)]. A 30 grooves/mm reflecting grating blazed at 8 µm was selected for the LWIR spectral region. The entrance and exit slit width was set to 1 mm. LIBS spectra were collected with an integration time of 15 µs and a gate delay of 5 µs. Longer delay and integration time settings were found to produce a rather large dark current background using this detection system. Figure 8 shows the atomic emissions from neutral potassium atoms centered at ~6.3, 7.4, and 8.5 µm, which were obtained by averaging a few shots (5-10 pulses) of the laser-induced plasma. The observed atomic emission features from both excitation sources exhibited similar LIBS K atomic spectra as those collected by a MCT single element system [17, 18]. It is noted that only the potassium atomic emission lines are observable from the potassium compound samples due to the limitation of the gate delay setting [24].
Fig. 8 LWIR LIBS emission spectra of potassium compounds pellets recorded by a MCT array detector system and integrated over only 5-10 laser pulses.
The readout integrated circuit MCT linear array detection system was recently developed by scientists/engineers at Brimrose Corporation of America [23, 24]. The linear MCT array detector is coupled with readout integrated circuits (ROIC) and it has 332 elements along the direction of dispersion. The dimension of each MCT pixel was 50 µm × 50 µm. The single line resolution limit of this LWIR LIBS detection system, the FWHM of a single narrow emission line, is around 76 nm. The array detector is capable of rapidly capturing (~1-5 second) a broad spectrum of LIBS emissions in the LWIR from 5.6 to 10 µm. The LWIR LIBS emission spectra covering the entire 5.6 to 10 µm region can be acquired from just a single laser-induced micro-plasma or by averaging a few single laser-induced micro-plasmas. More detailed descriptions of this MCT linear array detector is discussed in our previous work [23, 24].
Figure 9 illustrates the LWIR LIBS spectra of KNO3 pellets between 5.6 to 10 µm using the ROIC MCT linear array detection system with 1.064 µm and eye-safe laser 1.574 µm excitations at five different delay times. It was observed that LWIR LIBS studies using an eye-safe pump laser reproduced atomic and molecular LWIR LIBS spectra as previously observed under 1.064 µm laser excitation. In the early delay time, LWIR LIBS spectra of KNO3 pumped by laser wavelengths at 1.574 µm and 1.064 µm both shown strong atomic emission featuresfrom potassium (K) atoms and quickly decayed with time. The NO3 molecular vibrational emission features could be readily identified in LWIR LIBS spectra of both pumping laser wavelengths and lasted much longer than the atomic features. Just as observed elsewhere, LIBS emission intensity is smaller with 1.574 µm pumping than with 1.064 µm pumping.
Fig. 9 LWIR LIBS emission spectra of KNO3 pellets over the spectral range between 5.6 to 10 µm using the ROIC MCT linear array detection system with a (a) 1.064 µm and (b) eye-safe laser 1.574 µm excitation.
4. Conclusions
Eye-safe laser excitation is of critical importance in many LIBS applications such as standoff detection of hazardous chemicals. LWIR LIBS, with recent developments and advances, has shown to have great potential to be a potent analytical technique. The combination of atomic emission signatures derived from conventional UV-Vis LIBS and fingerprint signatures of intact target molecular entities determined from LWIR LIBS should make LIBS a more powerful spectral tool for chemical detection and analysis. Comparative LWIR LIBS studies were performed using the widely-used 1064 nm output of an Nd: YAG laser and the 1.574 µm eye-safe laser output from an Optical Parametric Oscillator (OPO) system. Several condensed-phase inorganic energetic materials and a chemical warfare agent simulant were tested in bulk pellet forms and as thin films deposited on asphalt substrates using three LWIR LIBS detection systems known to date. It was observed that LWIR LIBS studies using an eye-safe pump laser generally reproduced atomic and molecular IR LIBS spectra as previously observed under 1.064 µm laser excitation despite differences in amount of material ablated into the plasma volume, plasma temperature, and electron density of the laser-induced plasma. Mostly, the overall LWIR LIBS emission intensity is just slightly reduced when employing a 1.574 µm laser compared to 1.064 µm pumping for all studied materials. The LOD of eye-safe LIBS detection system therefore is expected to be close to that of 1.064 µm pumping LIBS detection system. With improvements on the MCT array detector suggested in the previous LWIR LIBS studies [23,24], one should be able to further reduced the LOD to sub-microgram (solid) and sub-microliter (liquid) per square centimeter range for LWIR LIBS detection system employing either a 1.574 µm eye-safe laser or a 1.064 µm laser as the excitation source.
Funding
National Science Foundation (NSF) (HRD-1137747); Army Research Office (ARO) W911NF15-1-0050; Small Business Technology Transfer (STTR) Phase III contract W911SR-C-0022 with funding from the Defense Threat Reduction Agency, project CB4059.
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